Lanthanide(iii)/actinide(iii) differentiation in coordination of azine molecules to tris(cyclopentadienyl) complexes of cerium and uranium

Abstract

Reaction of azine molecules L with the trivalent metallocenes [M(C₅H₄R)₃] (M = Ce, U; R = Bu^t, SiMe₃) in toluene gave the Lewis base adducts [M(C₅H₄R)₃(L)] (L = pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M = U and L = 3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e. [M(C₅H₄Bu^t)₃(L)] (L = pyridine, picoline) and [M(C₅H₄SiMe₃)₃(L)] (L = pyridine, lutidine, pyrimidine and dimethylpyrazine), the M–N and average M–C distances are longer for M = Ce than for M = U; however, within a series of azine adducts of the same metallocene, no significant variation is noted in the M–N and average M–C distances. The equilibria between [M(C₅H₄R)₃], L and [M(C₅H₄R)₃(L)] were studied by ¹H NMR spectroscopy. The stability constants of the uranium complexes, K_UL, are greater than those of the cerium counterparts, K_CeL. The values of K_ML are much greater for R = SiMe₃ than for R = Bu^t and a linear correlation is found between the logarithms of K_ML and the hydrogen-bond basicity pK_HB scale of the azines. Thermodynamic parameters indicate that the enthalpy–entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C₅H₄R)₃] and [U(C₅H₄R)₃] with L show that the selectivity of L in favour of U(III) increases with the π donor character of the metallocene and is proportional to the π accepting ability of the azine molecule, measured by its reduction potential.